Often, in an office, for example, a monitor is switched on in the morning and again switched off for the night. The monitor is thus on for the whole day and consumes energy even though its effective operation time may be just a fraction of the length of the working day. In some offices, the monitors are not shut down at all between working days. Therefore, several different solutions have been developed to reduce the power consumption of display units and monitors.
A known solution, which is aimed at prolonging the life of the picture tube rather than saving energy, is to use a computer program which blanks the screen when the system has not been used for a while. The main purpose of such a function is to prevent unchanging patterns from being burnt into the fluorescent material of the picture tube. At its simplest, this kind of function turns off the video signal from the display adapter.
Considerable energy savings are achieved with a monitor arrangement that switches the monitor off when a certain period has elapsed during which the system has not been used. This can be achieved e.g. using a circuit which recognizes that the video signal is missing and, for the duration of the signal absence, turns off the power for those parts of the monitor that consume the most power. With this arrangement, the power consumption of a monitor can be reduced to 5 to 8 W in the power saving state.
The video electronics standards association, VESA, has specified a display power management signaling (DPMS) system based on monitor synchronization signals. In the DPMS system, a monitor has four different states:
ON, which corresponds to the normal operation of the monitor; PA1 Stand-by, wherein the screen is blanked, for example, thus achieving a small decrease in power consumption; PA1 Suspend, wherein a major part of the monitor functions are switched off, and; PA1 OFF, where almost all functions of the monitor are switched off. PA1 power is supplied from the mains to the power supply secondary in the normal manner by means of the main power supply unit, PA1 power is supplied from the mains to the power supply secondary by means of passive components such as capacitors, and PA1 a separate power source is used.
The most significant power savings are achieved in the Suspend and OFF states.
In the DPMS system the monitor is driven to the desired state on the basis of vertical and horizontal synchronization pulses. The monitor has to be able to read the desired state from the vertical and horizontal synchronization pulse levels and to switch to the operating level specified by the synchronization pulses. The table below shows the synchronization pulse states corresponding to the various states.
______________________________________ Power Horizontal Vertical saving state sync pulses sync pulses ______________________________________ ON Yes Yes Stand-by No Yes Suspend Yes No OFF No No ______________________________________
In the table, `Yes` means that the frequency and pulse ratio of the input signal exceed a limit value defined in the DPMS system. Similarly, `No` means the frequency and pulse ratio of the input signal are below the limit value in question.
In a solution according to the DPMS system, the circuit reading the synchronization signals has to be able to measure the frequency and pulse ratio of the synchronization pulses, if required. The solution also has to be able to verify the current situation to avoid errors when the display adapter changes resolution, for example. In addition, energy is needed for reading the synchronization pulses and that energy cannot be taken from the display adapter, due to the technology applied. A common way of dealing with the synchronization pulse reading and monitor control is to use the monitor's microprocessor.
A solution widely used with the DPMS system is a so-called soft power switch. That means a device does not contain an actual mains power switch to switch off the power, but that switch is replaced by a switch located in connection with the processor. By means of that switch the device can be driven to the OFF state irrespective of the synchronization signal levels. To the user, the OFF state appears as if the device was switched off by a mains power switch.
At the moment there are three types of solutions for supplying power to the secondary of the power supply unit during the extreme power saving state, or the OFF state:
In a first version of the solution applying the main power supply the secondary of the power supply includes switching elements that disconnect parts of the system that consume power. The disconnection can be done by switching off the control signals or the operating voltages of the parts. In the suspend state, high-power blocks are disconnected. In the OFF state, all parts except the microprocessor or a corresponding circuit reading the synchronization signals and controlling the device are disconnected. In this kind of solution, the power supply unit of the device operates all the time and produces continuous stabilized operating voltages for the secondary. The advantage of the solution is its simplicity, but the efficiency of the power supply is poor. In addition, the solution requires many switching elements if there are several operating voltages.
In a second version of the solution applying the main power supply the operating voltages of the secondary are stabilized in the OFF state to a substantially lower level than in a normal operating situation, thus preventing the operation of the circuits loading the secondary and reducing the power consumption of the circuits to a low level. In practice, this is normally done by stabilizing a high stabilized operating voltage of the secondary, say, 150 volts, which is used for generating the other operating voltages of the secondary, to a level of about 8 volts. The operating voltage (+5 V, for example) for the processor is obtained from the reduced operating voltage by means of a regulator. The stabilization of voltages to a lower level in this version corresponds to the switches of the first version. In addition to the voltage reduction element it needs a switch that connects the reduced secondary voltage as the operating voltage for the processor for the duration of the power saving state. The advantage of the solution is still its simplicity, and at the same time the efficiency of the power supply is slightly improved because the voltage amplitudes generated in the power supply are smaller than in the first solution. The complexity of the circuit does not depend on the number of operating voltages as all operating voltages are reduced at the same time.
In a third version of the solution applying the main power supply, the operation of the power supply is not continuous but energy is supplied to the secondary in pulses. This further improves the efficiency of the power supply. In this kind of system, the operating voltages of the secondary are not stable during the power saving state but they contain oscillation at the pulse frequency. However, the processor is supplied a sufficient amount of operating voltage all the time and its operation is not interrupted.
The second basic solution uses passive components such as capacitors to transfer power from the mains to the secondary. The capacitive current of the capacitors is rectified and filtered in the secondary into a DC voltage. In order for the obtained power to be sufficient for ordinary processors, the capacitance of the capacitors must be high, which means that their physical size and the costs will increase.